1. Field of the Invention
The present invention relates to a novel catalyst for purifying an exhaust gas, catalyst which comprises Au.
2. Description of the Related Art
As an exhaust-gas-purifying catalyst for purifying an exhaust gas emitted from an automobile engine, a catalyst has been used widely which includes a porous oxide support, such as alumina, loaded with a noble metal, such as Pt, Rh and Pd. Recently, a research has been developed on a catalyst in which Au is used instead of a noble metal, such as Pt, Rh and Pd. For example, there is a catalyst, in which Au particulates are loaded on a support, such as alumina. The catalyst exhibits a good oxidizing activity for an inflammable gas. However, the catalyst has a problem in that the Au particulates exhibit low heat resistance. Hence, a variety of measures are taken in order to improve the heat resistance.
For instance, in Japanese Unexamined Patent Publication (KOKAI) No. 4-235,742, as a catalyst for decomposing aldehyde in an exhaust gas at a low temperature, there is disclosed an aldehyde-decomposing catalyst for purifying an exhaust gas, catalyst which includes: a composite oxide expressed by a chemical formula, Ce1xe2x88x92xZrxCu1xe2x88x92yAuyO3xe2x88x92z, in which xe2x80x9cxxe2x80x9d is 0.1 or 0.2, xe2x80x9cyxe2x80x9d falls in the range of from 0.02 to 0.04 and xe2x80x9czxe2x80x9d is 1.0 or less; an activated alumina containing cerium; and an oxide having an oxygen storage ability.
It is therefore an object of the present invention to improve the heat resistance of Au and to inhibit the purifying performance after a durability test from decreasing by making a structure of a catalyst different from that of the catalyst disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 4-235,742.
In an aspect of the present invention carrying out the aforementioned object, an exhaust-gas-purifying catalyst comprises: a composite oxide including gold (Au) and having crystalline unit cells in every one of which an Au ion is disposed.
In a further aspect of the present invention embodying the present exhaust-gas-purifying catalyst, an exhaust-gas-purifying catalyst comprises the composite oxide including at least one metal xe2x80x9cMxe2x80x9d selected from the group consisting of alkaline-earth metals and expressed by a chemical formula, Au2M5O8.
In a further aspect of the present invention embodying the present exhaust-gas-purifying catalyst, an exhaust-gas-purifying catalyst comprises the composite oxide including at least one metal xe2x80x9cXxe2x80x9d selected from the group consisting of rare-earth elements, at least one metal xe2x80x9cYxe2x80x9d selected from the group consisting of alkali metals, and expressed by a chemical formula, X2Au0.5Y0.5O4.
The exhaust-gas-purifying catalyst of the present invention is good in terms of the durability of the catalytic activity of Au, and can maintain high purifying performance from an initial stage to a final stage after a durability test.
Having generally described the present invention, a further understanding can be obtained by reference to the specific preferred embodiments which are provided herein for the purpose of illustration only and not intended to limit the scope of the appended claims.
The present exhaust-gas-purifying catalyst comprises the composite oxide which includes Au, and has crystalline unit cells in every one of which an Au ion is disposed. Namely, since the Au exists in a state of the ions, the present exhaust-gas-purifying catalyst exhibits a good catalytic activity. Further, this exhaust-gas-purifying catalyst is extremely superb in terms of the heat resistance. It is believed that these advantages result from the arrangement that the Au is present in such a manner that it is surrounded by the other elements, and that it is accordingly inhibited from moving.
Note that, in the conventional catalyst expressed by the chemical formula, Ce1xe2x88x92xZrxCu1xe2x88x92yAuyO3xe2x88x92z, in which xe2x80x9cwxe2x80x9d is 0.1 or 0.2, xe2x80x9cyxe2x80x9d falls in the range of from 0.02 to 0.04 and xe2x80x9czxe2x80x9d is 1.0 or less, the Au content is so insufficient that the Au ion is not disposed in every one of the crystalline unit cells. Thus, the conventional catalyst is structurally different from the present exhaust-gas-purifying catalyst, and operates differently from the present exhaust-gas-purifying catalyst in which the Au mainly purifies an exhaust gas.
In the aforementioned present exhaust-gas-purifying catalyst, it is possible to composite a variety of metals as a metallic element other than the Au. However, due to the production reasons, it is preferred that a metal is selected from the group consisting of alkaline-earth metals, the group consisting of alkali metal or the group consisting of rare-earth elements. For example, when an alkaline-earth metal (M) is composited in the composite oxide, the structure, which is turned into the crystal including an Au ion in every one of the crystalline unit cells, is expressed by a chemical formula, Au2M5O8.
Further, it is possible to composite a plurality of metals other than the Au. For instance, when both of a rare-earth element (X) and an alkali metal (Y) are composited in the composite oxide, the structure, which is turned into the crystal including an Au ion in every one of the crystalline unit cells, is expressed by a chemical formula, X2Au0.5Y0.5O4.
It is possible to composite an alkaline-earth metal doped with an alkali metal or a rare-earth element. It is also possible to composite a rare-earth element doped with an alkali metal or an alkaline-earth metal. It is further possible to composite an alkali metal doped with an alkaline-earth metal or a rare-earth element.
As for the alkaline-earth metal, which is used preferably in the present exhaust-gas-purifying catalyst, Be, Mg, Ca, Sr and Ba can be exemplified, and it is possible to composite at least one of these elements. Among them, Sr and Ba are especially preferred. As for the rare-earth element, it is preferable to use at least one member selected from the group consisting of La, Ce and Nd. As for the alkali metal, it is preferable to use at least one member selected from the group consisting of Li, Na, K, Rb and Cs.
In the present exhaust-gas-purifying catalyst, it is, of course, possible to composite only Au. It is also possible to composite Au doped with a noble metal, such as Pt, Pd and Ag.
The present exhaust-gas-purifying catalyst can be used by loading it on a porous oxide, such as activated alumina, alumina, zirconia, silica and titania. The present exhaust-gas-purifying catalyst can be produced by mixing the powders of the aforementioned metals or metallic compounds in a solid phase and burning them. Alternatively, oxide precursors can be precipitated from a mixture solution including the aforementioned metals by the co-precipitation method or the sol-gel process, and they can be burned to make a composite oxide.